This invention relates to a high strength flooring composition. More specifically, it relates to a flooring composition made using beta-calcined calcium sulfate hemihydrate, a dispersant and a modifier.
Both gypsum and cement are well known as construction materials. Gypsum is the principal component of wallboard, where is it faced with paper to provide strength and a smooth surface. Cement is used in various applications where its water resistance and hardness are important, such as in concrete structures. Cement is also used in building applications where its hardness and water resistance are important.
Gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Plaster of Paris is also known as calcined gypsum, stucco, calcium sulfate semihydrate, calcium sulfate half-hydrate or calcium sulfate hemihydrate. Synthetic gypsum, which is a byproduct of flue gas desulfurization processes from power plants, may also be used. When it is mined, raw gypsum is generally found in the dihydrate form. In this form, there are approximately two water molecules of water associated with each molecule of calcium sulfate. In order to produce the hemihydrate form, the gypsum is calcined to drive off some of the water of hydration by the following equation:CaSO4.2H2O→CaSO4.1/2H2O+3/2H2O
When mixed with water, the hemihydrate rehydrates to form an interlocking matrix of interlocking dihydrate crystals. Gypsum hydration occurs in a matter of minutes or hours compared to several days for cement. This makes gypsum an attractive alternative for many applications if sufficient hardness and strength can be achieved in the gypsum.
Calcium sulfate hemihydrate can produce at least two crystal forms during calcination. Alpha-calcined gypsum is made by a continuous process or lump rock process whereby the calcium sulfate dihydrate is calcined under pressure. The alpha-calcined gypsum forms less acicular crystals than beta-calcined gypsum, allowing the crystals to pack tightly together, making a denser and stronger plaster. The crystal morphology allows water to flow easily between the crystals, requiring less water to form a flowable slurry. More elongated crystals are characteristic of the beta-calcined gypsum. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the calcined gypsum. In applications where hardness is important, alpha-calcined gypsum is usually preferred, despite higher cost and limited availability.
When choosing a calcined gypsum for an application, beta-calcined gypsum is often selected due to its ready availability and its reduced cost. Because beta-calcined gypsum is also more common, it can incur reduced shipping and storage costs than the alpha form. However, the crystal structure makes it difficult to make a strong, dense gypsum because more water is needed to produce a sluny of a given fluidity. When the gypsum is dry, voids once occupied by water remain in the crystal matrix, weakening it and producing a product having less strength than gypsum made with smaller amounts of water. A low-water gypsum slurry is particularly useful in an application such as a poured floor, where strength is important. Gypsum-based flooring is advantageously used where rapid set of the floor is required.
Dispersants are known for use with gypsum that help fluidize the mixture of water and calcium sulfate hemihydrate so that less water is needed to make a flowable slurry. However, these dispersants have several disadvantages. Polycarboxylate dispersants can be severely retardive, significantly reducing the rate of the setting reaction. Increases in the setting time can disrupt the manufacturing process of the floor. Lengthened set times result in delays in finishing and using the floor. Accelerators that may be added to overcome the retarding effect of the dispersant may be rendered less effective than in compositions where no dispersant is used.
Despite the large amount of prior art to polycarboxytate dispersants, it is difficult to predict the effect of any particular compound on the products produced with it. Polycarboxylates are generally known to improve fluidity in cement. This does not necessarily mean that the polycarboxylate will produce the same result in gypsum products. Gypsum and cement form different crystal patterns that may disperse differently in a polycarboxylate solution. The set times of these hydraulic materials are very different, making the retardive effects of some polycarboxylates that are negligible in cement critical to the set of a gypsum wallboard. There are even variations within the realm of gypsum products, with some polycarboxylates being effective for certain gypsum sources and not others. The complete lack of predictability of polycarboxylate efficacy in gypsum or cement makes it difficult to make a low-water product.
Further, retardation of the set times by the use of large amounts of polycarboxylate dispersants cannot always be overcome by the addition of set accelerators. When high dosages of polycarboxylates are used to make a flowable slurry at low water to stucco ratios, the set time can be delayed enough to require changes in how the product is handled, often resulting in inefficiencies in the manufacturing process.
Surface water balance can be difficult to achieve in hydraulic products. Sand or other aggregates in the sluny can fall out of the suspension and sink to the bottom of the mixture. Water can also come out of the suspension, rising to the top of the slurry. If too much water bleeds from the slurry, the surface is weakened because of voids left from drying. However, if the surface is too dry, there is insufficient water to hydrate the calcined gypsum. This leaves a powder on the surface of the flooring that makes it more difficult to adhere finished floor goods such as vinyl or ceramic tile, without additional preparation. Wear resistance of the floor is also reduced. When used in a poured floor, for example, it is advantageous to have a thin film of water on the surface of the slurry. This water makes sure the surface is hydrated and also aids in finishing the surface. Some dispersants rise to the top of the slurry with the water, leaving the dispersant unevenly distributed in the bottom of the fluid. If the dispersant rises into the water film on top of the slurry, the surface water can become sticky, leaving a poor surface that is difficult to finish.